There appears to be some hope for Java developers on the Mac platform... it is now possible, thanks to Landon Fuller, to build Java 1.6.0_03 on Mac OS Leopard.
I can confirm that this works, at least for non-GUI code. I successfully compiled and ran a 64-bit VM. 32-bit was more problematic; all attempts at building caused a JVM crash which I was not able to overcome.
Details and benchmarks after the fold.
Recently I have been dealing with the disconnect between Mac OS X and Java 6, specifically that Java 6 is not available. This is not intended to be yet another diatribe on the evils (or not) of Apple's approach to this. Rather, it has gotten me motivated to work on some tools to make creating cross-platform GUI code in Java a bit easier to manage.